1. Field of the Invention
The present invention relates to a novel photosensitive phosphorylated phenol-formaldehyde resin and method for preparing the same. The phosphorylated phenol-formaldehyde resin is curable under UV light and soluble in alkaline solution; moreover, it exhibits good flame-retardant and adhesive properties.
2. Description of the Prior Art
Photosensitive resins have been widely used in coating materials, adhesives, printing inks, printed resin boards, printed circuit boards and semiconductors thanks to their characteristics, such as non-solvent-producing, high production efficiency due to short curing time, low investment costs due to small equipment volume needed for manufacturing, and others. Recently, the processing conditions for manufacturing circuit boards and electronic devices have gotten more strict, creating the need for a photosensitive resin with more integrated functions. The important functions concerned are flame retardancy, and good adhesion with metal or other substrates.
The flame retardant property of conventional photosensitive resin composition is generally achieved by adding halogen compounds. This method, however, has its drawbacks. Using brominated epoxy resin as an example, although there is a certain degree of flame retardancy, it produces large amounts of corrosive and toxic hydrogen bromine gas. In addition, dioxin may be generated.
Another conventional flame retardant is phosphoric compound. It has low toxicity, is less smoky, has better compatibility with resin and good workability, and requires lower dosage for use. Moreover, it improves the adhesion between resin and metals or other substrates. Its use is, therefore, an inevitable trend.
In Japanese Patent Application Laid-Open No. 9-328571, an aromatic organic phosphate compound is added to obtain a resin composition with good flame retardance, and good resistance to heat and shock.
Also, in Japanese Patent Application Laid-Open No. 2-153902, a phosphorous-containing acrylic monomer is added to an epoxy acrylate photosensitive resin composition to improve the adhesive strength of the resin.
In Japanese Patent Application Laid-Open No. 2-003454, adhesion is improved by adding a phosphate photosensitive monomer to an epoxy acrylate photocurable resin composition, which is used as a solder or plating resist in the manufacture of printed circuit boards.
In the above-mentioned patent applications, by adding phosphorous-containing photosensitive monomers to the resin composition, the viscosity of the resin is lowered, and the adhesive strength between the resin and substrate is improved. However, the phosphorous compound of a low molecular weight easily migrates and vaporizes to degrade the physical properties and flame retardancy. In addition, since the phosphorous-containing photosensitive monomer is irritating, it cannot be added in a large amount. Consequently, its application as a flame retardant is rather restricted.
The following articles relate to phosphorous-containing epoxy acrylate photosensitive resins. U.S. Pat. No. 5,512,607 discloses reacting a cresol novolak type epoxy resin with a carboxylic acid having an ethylenically unsaturated group to esterify some of the epoxy groups, and then reacting the unreacted epoxy groups with a tertiary amine as well as phosphoric acid to form a photosensitive resin containing quaternary ammonium phosphate groups. In Japanese Patent Application Laid-Open No. 7-157711, a water-soluble coating composition is prepared by reacting a phosphorylated epoxy resin with a polymerizable monocarboxylic acid having an ethylenically unsaturated bond.
The object of the present invention is to provide a novel photosensitive phosphorylated phenol-formaldehyde resin which is curable under UV light and soluble in alkaline solutions.
Another object of the invention is to provide a photosensitive phosphorylated phenol-formaldehyde resin which is flame retardant and promotes adhesion. Thus, the environmental pollution problems caused by conventional halogenated flame retardant will be eliminated, and the adhesion between the photosensitive resin and substrate can be improved.
To achieve the above-mentioned objects, the novel photosensitive phosphorylated phenol-formaldehyde resin of the invention is characterized by having at least two phosphate groups in its molecule, and each phosphate group is coupled with at least one photosensitive group. The photosensitive phosphorylated phenol-formaldehyde resin is prepared by reacting phenol-formaldehyde resin with phosphorus oxychloride (POCl3) to form a phosphorylated phenol-formaldehyde resin; then reacting the resin with a photosensitive compound having an hydroxyl group and at least one ethylenically unsaturated bond to proceed dehydrochlorination, so that a photosensitive group is introduced into the resin. Finally, the unreacted phosphorochloridate group is hydrolyzed into a phosphoric group.
According to the preparation method of the invention, a phenol-formaldehyde resin with repeating unit (I) and POCl3 are reacted to form a phenol-formaldehyde resin with repeating unit (II), wherein R1 represents C1-6 alkylene or C6-10 alicyclic ring; and R2 represents hydrogen or methyl. 
The reaction ratio between POCl3 and the phenol-formaldehyde resin is not specifically restricted. However, it is preferable to set the ratio of moles of POCl3 to the phenol equivalent of the phenol-formaldehyde resin at 0.5-1.3.
Moreover, the phenol-formaldehyde resin used in this invention is not specifically limited, as long as it can be phosphorylated by POCl3. Examples of the phenol-formaldehyde resins are: phenol series phenol-formaldehyde resin, formaldehyde series phenol-formaldehyde resin and alicyclic series phenol-formaldehyde resin.
Next, the phosphorylated phenol-formaldehyde resin with repeating unit (II) is reacted with a photosensitive compound (III) having an hydroxyl group and at least one ethylenically unsaturated bond in its molecule to proceed dehydrochlorination. Thereby, a phosphate group having an ethylenically unsaturated bond is coupled to the phenol-formaldehyde resin as shown in formula (IV), 
wherein, R1 and R2 have the meaning described above; R3 is C2-6 straight-chain or branched alkylene and optionally substituted with heteroatoms or aryl groups; Y is a functional group having an ethylenically unsaturated bond; and x is 0 or 1.
The reaction ratio between the photosensitive compound (III) and the phenol-formaldehyde resin (with repeating unit (I)) is not specifically restricted. However, it is preferable to set the ratio of moles of photosensitive compound (III) to the phenol equivalent of the phenol-formaldehyde resin at 0.5-1.5.
The photosensitive compound (III) used in this invention is not limited, as long as it contains an hydroxyl group and at least one ethylenically unsaturated bond in its molecule. For example, the photosensitive compound can be a compound containing an hydroxyl group and at least one acrylate group in each molecule, or a compound containing an hydroxyl group and at least one methacrylate group in its molecule. Preferable photosensitive compounds include hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, and hydroxybutyl methacrylate.
Finally, the unreacted phosphorochloridate groups in the above phenol-formaldehyde system are hydrolyzed into phosphoric groups to obtain the phosphorylated phenol-formaldehyde resin of the invention, which contains repeating unit (V): 
wherein, R1, is C1-6 alkylene or C6-10 alicyclic ring; R2 is hydrogen or methyl; R3 is C2-6 straight-chain or branched alkylene and optionally substituted with heteroatoms or aryl groups; Y is a functional group having an ethylenically unsaturated bond; and x is 0 or 1.
In addition, the reactant system may contain a phenol-formaldehyde resin with repeating unit (I) and a phosphorylated phenol-formaldehyde resin with repeating unit (VI) that is not coupled with a photosensitive group: 
wherein R1, and R2 have the meaning described above.
The photosensitive phosphorylated phenol-formaldehyde resin of the invention contains a functional group having an ethylenically unsaturated bond (such as acrylate group or methacrylate group). Hence it is UV-curable as the conventional photosensitive resins are. Moreover, the phosphate group contained in the resin is able to promote the adhesion between resin and substrate, and to improve the flame retardancy after the resin is cured. The limiting oxygen index (LOI) of the photosensitive resin composition of the invention is generally greater than 26, and a value of greater than 45 can be obtained in a preferred embodiment.
In the preferred embodiments of the invention, Y represents an acrylate group or a methacrylate group. In the most preferred embodiments of the invention, R1 is CH2; R2 is hydrogen or CH3; R3 is CH2 or CH2CH2; the number of the repeating unit (I) of the phenol-formaldehyde resin is 1-10; the number of the repeating unit (V) of the phosphorylated phenol-formaldehyde resin is 1-5.
The photosensitive phosphorylated phenol-formaldehyde resin of the invention can be mixed with a photoinitiator to give a resin composition that is curable under UV light. Suitable photoinitiators include benzophenone, benzoin, benzoin alkyl ether, and acetophenones. The amount of the photoinitiator is about 0.1-25 wt % of the photosensitive resin, preferable 0.1-10 wt % of the resin.
The novel photosensitive phosphorylated phenol-formaldehyde resin can be used alone or with other photosensitive resins. Therefore, the above photosensitive composition may further comprise a second photosensitive resin different from the photosensitive phosphorylated phenol-formaldehyde resin, such as epoxy acrylate resin. Moreover, other additives such as thermosetting resin and thermocuring initiators can be added.
Diluents can be added to adjust the viscosity of the resin composition. Suitable diluents include reactive monomers such as trimethylolpropane triacrylate, pentaerythritol tetracrylate, and tetraglycol diacrylate.
In addition to the above-mentioned main components, other conventional additives can be present in the resin composition of the present invention. Such additives include thermal stabilizers, antioxidants, photo-stabilizers, lubricants, colorants (such as dyes, pigments), various types of fillers and reinforcing agents.
The phosphorous-containing resin composition can be used as electronic device molding compounds, solder masks for printed circuit boards, photosensitive insulating dielectric materials and photo-curable coating materials for all sorts of substrates.
The scope of the invention also includes cured product of the photosensitive resin composition, which can be either photo-cured or thermo-cured. By using the phosphorous-containing photosensitive resin composition of the invention, the cured product has better flame retardancy and adhesion.
The foregoing description of the preferred embodiments of this invention has been presented for purposes of illustration and description. Obvious modifications or variations are possible in light of the above teaching. The embodiments were chosen and described to provide the best illustration of the principles of this invention and its practical application to thereby enable those skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.
The formulas of the compounds used in the examples follow: 